Question

In which one of the following pairs the radius of the second species is greater than that of the first?

• A. Na, Mg
• B. $O^{2-}, N^{3-}$
• C. $Li^{+}, Be^{2+}$
• D. $Ba^{2+}, Sr^{2+}$

Hint:

In which one of the following pairs the radius of the second species is greater than that of the first?

Answer 1

The Correct Option is B

Step 1: Concept
$O^{2-}$ and $N^{3-}$ are isoelectronic species (same number of electrons).
Step 2: Analysis
For isoelectronic ions, as the negative charge increases, the ionic radius increases.
Step 3: Conclusion
The electrons are held less tightly by the nucleus in $N^{3-}$, making its radius greater than $O^{2-}$.
Final Answer: (B)